1. Field of Invention
The present invention relates to a liquid crystal composition, a liquid crystal element, a reflective display material, a light-controlling material, and method of preparing a liquid crystal composition.
2. Description of the Related Art
Display devices of a guest-host system in which display is carried out by change in the orientation state of a dichroic dye dissolved in a liquid crystal are capable of bright display, and are expected to serve, not only as reflective display devices, but also as systems adapted to light-controlling applications.
In this guest-host system, the dichroic dye has one light absorption axis and absorbs only light that oscillates in the direction of the light absorption axis. If the orientation of the dichroic dye can be changed to match the movement of the liquid crystal by an electric field, the direction of the light absorption axis can be controlled, and the light absorption state of a cell can be changed. Furthermore, it is known that the display contrast of a guest-host system liquid crystal element is determined by an order parameter of the dichroic dye contained in a liquid crystal composition constituting a liquid crystal layer, a host liquid crystal, or a cell structure.
Generally, when a liquid crystal composition in which a nematic liquid crystal and a dichroic dye are combined is provided between substrates that have been subjected to orientation processing, since the nematic liquid crystal exhibits a uniaxial orientation state, the dichroic dye can only absorb linearly polarized light of one of the axes. Accordingly, half of the light is transmitted and a display contrast ratio cannot be raised.
Further, even in a case where a liquid crystal composition in which a nematic liquid crystal and a dichroic dye are combined is injected between substrates that have not been subjected to orientation processing, the nematic liquid crystal exhibits a multi-domain state, and the dichroic dye can only absorb linearly polarized light of one of the axes. Accordingly, half of the light is transmitted and a display contrast ratio cannot be raised.
Meanwhile, a phase transition guest-host system which utilizes a chiral nematic phase due to combination with a chiral dopant so that the dichroic dye can absorb light of all orientations has been proposed (D. L. White and G. N. Taylor, J. Appl. Phys., Vol. 45, pp. 4718 (1974)). In this system, bright display that does not use a polarization plate becomes possible.
However, in the phase transition guest-host system, since incident light rotates following a helical structure (chiral nematic phase) (wave guide), there is the problem of a reduction in the amount of light absorbed by the dichroic dye. In order to raise the light absorption amount, it is necessary to make the refractive index anisotropy Δn of the host liquid crystal as small as possible, or shorten the helical pitch length to an extent that incident light cannot follow.
Incidentally, in a non-absorption (colorless) mode (normally white) at a time when a voltage is not applied, it is preferable that the liquid crystal has a homeotropic orientation. However, when the helical pitch length is shortened as discussed above for the purpose of raising the light absorption amount in a homeotropic orientation, the twisting power of the liquid crystal overcomes the angling power of the orientation film, and there is a problem in that, as a result of attaining a focal conic state (in which the helical axis is parallel to the substrate), the transmission rate of the non-absorption state is lowered.
It has been reported that a helical pitch length that can maintain a homeotropic orientation correlates with an elastic constant ratio K33/K22 of the host liquid crystal, and that the larger this value of the host liquid crystal is, the more the helical pitch length can be shortened (for example, Physical Review A, Vol. 44, No. 12, pp. 8198-8210 (1991)). However, this report is based on logical calculation and does not disclose any concrete measures for realization.
Furthermore, it has been considered that, in order to change the elastic constant ratio K33/K22, the only option is to change the host liquid crystal itself. As a result, it has been difficult to satisfy all of the various properties (refractive index anisotropy Δn, liquid crystal temperature range, order parameter and viscosity) that are necessary for the guest-host system, in addition to the elastic constant ratio K33/K22, and thus a new method has been demanded.